I,m using a Bowman ring and this gent and his products are a class act.It alone increases the compression of the engine somewhat since it overlaps the pin in the piston and seals the joint insread of just butting as the stock ring does.
Im using a B&B mount,a Bisson muffler,and the prop driver comes with the CH ignition.
Utilizing a used or heavy discount weeder,and allowing about $120 for the CH conversion you should be able to bring the cost of the engine in about $200.It isnt worth putting much more into this type of conversion when you consider an RCS 1.4 sells for $325.
Actually I only went as far as I did to prove to myself that a weeder conversion was viable for most 1.20 sized models and I assumed I could sell the CH unit used or use it on other engines.To really keep the costs down a shaved flywheel is the way to go and this will fly your 4/120 with some spirit but its a tight fit,weighs a lot more and is ugly as sin if you dont cowl it,which presents its own problems ,but is possible.There is no free lunch!
If I can get my new digicam working,Ill post a picture here of my installation in the next few days if that will help.
IF you havent already done so ,check out the weeder conversion part of WWW.rcfaq.com/home.htm
There is a Ryobi4/120 article buried there,on Marty Mammersmith,s home page.